Circulating grinding plant comprising a mill and a sifter

ABSTRACT

The aim of the invention is to create a compact circulating grinding plant comprising a product bed comminution roller mill or roller press and a sifter device, in which the problem of mechanically transporting the grinding product that is directed through the circuit is minimized. Said aim is achieved by a circulating grinding plant in which a static cascade sifter is disposed below the gap of the roller press while a subsequent sifter, particularly a dynamic rod basket sifter, is disposed above said roller press. The sifting product discharge area of the static cascade sifter is connected to the sifting air and sifting product inlet of the subsequent sifter via an ascending duct which pneumatically conveys the product.

The invention relates to a circulating grinding plant for the grindingof raw material, with a high-pressure roller mill or roller press withproduct bed comminution of the feed material and with a sifter device,comprising a static cascade sifter, to which at least one additionalsifter is subsequently added on the sifting air current side, whoseoversize material discharge is connected to the material feed of theroller press.

Pressure comminution of granular grinding product in the gap between twopress rollers is known in the manner that the individual particles ofthe grinding product are mutually crushed upon application of a highpressure in a material bed compressed between the two roller surfaces,so that in this case one speaks of the so-called product bedcomminution. The compression stress results partly in a particledestruction of the grinding product, and partly in the generation ofcracks in the particle interior and manifests itself visibly in theformation of agglomerates (so-called scabbing), which can bedeagglomerated or dissolved with comparably low expenditure of energy,so that this type of comminution is characterized overall by acomparably low specific power consumption.

In practice such a product bed roller mill or roller press isinterconnected with a sifter device to a circulating grinding plant. Forexample a circulating grinding plant is known both from DE-A-42 23 762FIG. 2 as well as from EP-B-0 650 763 with a roller press, staticcascade sifter and with a subsequent sifter. In the process the staticcascade sifter employed as preliminary sifter and the subsequent siftersubsequently added on the sifting air current side, which can be adynamic or a static sifter, are each combined into a compactconstructional unit. The raw material is fed into the grinding circuit,in particular by means of feeding to the roller press or by meansfeeding of to the preliminary sifter, and the finished fine processedmaterial is discharged out of the grinding circuit, to be precise bymeans of the sifter gas stream exiting the subsequent sifter. Thecascade sifter acts as a preliminary sifting chamber for the grindingproduct directed into the circuit as well as acting simultaneously as adeagglomerator for dissolution of the pressure scabs still present inthe recycle material so that a dynamic rod basket sifter subsequentlyadded as a subsequent sifter with it rotating turbo elements is nolonger loaded with coarse grain components. Both the coarse grainfraction from the static cascade sifter as well as the coarse grainfraction separated from the grinding product by the subsequent siftercan be re-circulated to the roller press for the purpose of repeatedproduct bed stress and further grinding. Depending on the recirculationquantities the rate of circulation in such a grinding circuit can behigh. High grinding circuit rates, i.e., great quantities of circulatingscabbing material or great quantities of a mixture of scabbing and rawmaterial cause a considerable expenditure for the mechanical transportof this material to the sifter device, whereby the transport device as arule consists of a bucket conveyor, which causes not inconsiderableinvestment and operating expenses.

The invention is based on the object of creating a compact circulatinggrinding plant comprising a product bed comminution roller mill orroller press and a sifter device in which the problem of mechanicallytransporting the grinding product that is directed through the circuitis minimized.

The invention solves this task with a circulating grinding plant withthe features of claim 1. Advantageous improvements of the invention arespecified in the dependent claims.

In the case of the invention's circulating grinding plant with rollerpress and sifter device at least one static cascade sifter is disposedbeneath the gap of the roller press, said cascade sifter whichsimultaneously deagglomerates and sifts the scab material coming fromthe roller press. A static or in particular a dynamic subsequent sifteris arranged above the roller press, in the case of the latter namedsifter type at least one rotatably arranged rod basket of a dynamic rodbasket sifter provided with turbo elements on the rotor periphery forsubsequent sifting of the grinding product directed through the circuit.The gritty material separated by the subsequent sifter, e.g., rod basketsifter is re-circulated by means of a penstock as a result ofgravitational effect for material feed of the roller press. The siftingproduct discharge area of the static cascade sifter is not transported bmeans of a mechanical conveying agent such as e.g. a bucket conveyor,but rather is pneumatically conveyed through an ascending duct to thesifting air and sifting product inlet of the subsequent sifter, inparticular dynamic rod basket sifter. As an ideal, 100% of the productof the circulating grinding plant is pneumatically conveyed through theascending duct from the static cascade sifter up to the subsequentsifter. This solution saves investment expenses, operating expenses andspace for the installation of an otherwise necessary mechanicalconveying agent such as a bucket conveyor.

For the case that a grinding product is treated in the invention'scirculating grinding plant in which the coarse grain fraction collectedin the lower region of the static cascade sifter is not fine enough tobe pneumatically conveyed from the sifting air current through theascending duct to the dynamic rod basket sifter, in accordance with anadditional feature of the invention a pulverizer in particular impactpulverizer that pulverizes the sifting coarse grain fraction or theremaining product agglomerates can be integrated in the lower region ofthe static cascade sifter, e.g. an impact hammer mill, whose rotor spinsthe sifter coarse grain fraction back up to the static cascade sifterafter further pulverizing for the purpose of additional pneumaticconveyance of the grinding product through the ascending duct to thesubsequent sifter. However, instead of this, there is also thepossibility of equipping the static cascade sifter with a coarse grainfraction discharge opening on its underside, said discharge openingbeing connected to the material feed of the roller press via an externalconveying agent.

The invention and its further features and advantages are described ingreater detail with the aid of the embodiments schematically representedin the drawing.

The drawing shows a compact circulating grinding plant with a productbed comminution roller mill or roller press 10 and with a sifter device.Raw material 11 that is to be ground is fed into the feed well of theroller press 10. The grinding product exits the gap pulverized andpartially agglomerated, i.e., pressed into scabs 12, whose percentage ofparticles already reduced to the desired cement fineness can berelatively high, e.g., 30% smaller than 90 μm.

A static cascade sifter 13 is arranged below the gap of the roller press10, with the scab material 12 being directed into the cascade sifter'sproduct feed opening above. The cascade sifter 13 exhibits a v-shapedshaft-shaped case with two sifting zone boundary partitions enclosed bythe sifter case and forming a sifting zone 14 between them which havesifting air 15 (or hot gas as a drying gas in the case of damp feedmaterial) flowing through them, said boundary partitions exhibitingcascade-like or louver-like baffle plates 16 a, 16 b arranged incliningdownward, whereby these two baffle plates and the sifting zone 14 lyingbetween them are arranged lying at an angle deviating from the verticallines.

When gravity causes the scab material 12 to fall from above to belowthrough the baffle plate cascades upon the impact of the productparticularly to baffle plates 16 a and 16 b and with the circulation ofthe product the product agglomerates are deagglomerated and the siftingproduct is simultaneously sifted in the cross-flow.

The cascade sifter 13 is closed at its underside. The sifting productdischarge of the cascade sifter 13 is pneumatically conveyed suspendedin the sifter exhaust 18 via an ascending duct 19 past the roller press10 to the sifting air and sifting product inlet of a subsequent sifterin the drawing of a dynamic rod basket sifter 20, whose pivoted rodbasket 21 equipped with turbo elements is arranged at the rotorperiphery with e.g., horizontal rotational axis above the roller press10. While the coarse grain fraction (grit) is discharged at theperiphery of the rotating rod basket 21 with a fineness e.g., 100% lessthan about 500 μm, said grain fraction which is re-circulated via apenstock 22 to the product feed well of the roller press 10, the flow ofsifting air/finished product represented by the arrow 23 is dischargedvia at least one of the two frontal areas of the rotating rod basket 21and directed through a cyclone centrifugal separator 25 for the purposeof separation of the fine finished product 24 at a finished productfineness e.g. 100% less than about 32 μm, whose exhaust air 26 isre-circulated via a ventilator 27 as circulating air in the staticcascade sifter 13.

There is also the possibility of taking the raw material 11, e.g., if itexhibits a high humidity, and instead of feeding it at the product feedwell of the roller press 10, feeding it at its product discharge andhence feeding it into the circulating grinding plant at the materialfeed of the static cascade sifter 13 and then using a hot drying gas asthe sifting air stream 15.

For the case that the coarse grain fraction gathering in the lowerregion of the static cascade sifter 13 is not fine enough to betransported by the sifting air flow 18 through the ascending duct 19 upto the dynamic rod basket sifter 20, a pulverizer, in particular impactpulverizers 28 such as e.g., an impact hammer mill that furtherpulverizes the sifter-coarse grain fraction or any product agglomeratescan be integrated in the lower region of the cascade sifter 13, saidhammer mill whose rotor keeps spinning the sifter coarse grain fractionback into the sifter until it is fine enough to be pneumaticallyconveyed above through the ascending duct 19.

The circulating grinding plant of the invention also allows for thepossibility of using a tube mill or ball mill or the like in place ofthe high pressure roller mill or roller press.

1. A circulating grinding plant for the grinding of raw material, with ahigh-pressure roller mill or roller press (10) with product bedcomminution of the feed material and with a sifter device, comprising astatic cascade sifter (13), to which at least one additional sifter (20)is subsequently added on the sifting air current side, whose oversizematerial discharge (22) is in connection with the material feed of theroller press (10), characterized by the following features: a) thestatic cascade sifter (13) is disposed beneath the gap of the rollerpress (10), b) the subsequent sifter (20) with its coarse graindischarge pipe (22) is disposed above the roller press (10), c) thesifting material discharge area (18) of the static cascade sifter (13)is connected to the sifting air and sifting product inlet of thesubsequent sifter (20) via an ascending duct (19) which pneumaticallyconveys the product.
 2. Circulating grinding plant according to claim 1,characterized by the fact that the static cascade sifter (13) exhibits av-shaped shaft-shaped case with two sifting zone boundary partitionsenclosed by the sifter case and forming a sifting zone (14) between themwhich have sifting air (15) or hot gas as a drying gas flowing throughthem, said boundary partitions exhibiting cascade-like or louver-likebaffle plates (16 a, 16 b) arranged inclining downward, whereby thesetwo baffle plates and the sifting zone (14) lying between them arearranged lying at an angle deviating from the vertical lines. 3.Circulating grinding plant according to claim 1, characterized by thefact that the subsequent sifter (20) subsequently added to the staticcascade sifter (13) is a dynamic rod basket sifter with at least onepivoted rod basket (21) equipped with turbo elements arranged at therotor periphery.
 4. Circulating grinding plant according to one ofclaims 1 through 3, characterized by the fact that in the lower regionof the static cascade sifter (13) a pulverizer is integrated, inparticular an impact pulverizer (28) which further pulverizes the siftercoarse grain fraction or product agglomerates.
 5. Circulating grindingplant according to claim 4, characterized by the fact that theintegrated pulverizer (28) is an impact hammer mill, whose rotor keepsspinning the sifter coarse grain fraction back into the static cascadesifter (13) for the purpose of further pneumatic conveyance of thegrinding product through the ascending duct (19) to the subsequentsifter (20).
 6. Circulating grinding plant according to one of claims 1through 4, characterized by the fact that the static cascade sifter (13)exhibits a coarse grain fraction discharge opening at its underside,said discharge opening which is connected to the material feed of theroller press (10) via an (external) conveying agent.
 7. A circulatinggrinding plant for a grinding of raw material, having a high-pressureroller mill or roller press providing product bed comminution of the rawmaterial and having a sifter device, comprising a static cascade sifterprovided with sifting air flow flowing from an inlet side to sifting aircurrent side, at least one additional sifter positioned on the siftingair current side, said additional sifter having an oversize materialdischarge connected to a material feed of the roller press, the staticcascade sifter being disposed beneath a gap of the roller press, thesubsequent sifter having a coarse grain discharge pipe being disposedabove the roller press, a sifting material discharge area of the staticcascade sifter being connected to one of a sifting air and a siftingproduct inlet of the subsequent sifter via an ascending duct providedwith an airflow averaged to pneumatically convey the product.
 8. Acirculating grinding plant according to claim 7, wherein the staticcascade sifter has a v-shaped shaft-shaped case with two sifting zoneboundary partitions enclosed by the sifter case and forming a siftingzone between them which have sifting air or hot gas as a drying gasflowing through them, said boundary partitions having louvered baffleplates arranged inclining downward, whereby these two baffle plates andthe sifting zone lying between them are arranged lying at an angledeviating from vertical.
 9. A circulating grinding plant according toclaim 7, wherein the subsequent sifter is a dynamic rod basket sifterwith at least one pivoted rod basket equipped with turbo elementsarranged at a rotor periphery.
 10. A circulating grinding plantaccording to claim 7, wherein in a lower region of the static cascadesifter an impact pulverizer is integrated which further pulverizes asifter coarse grain fraction or product agglomerates.
 11. A circulatinggrinding plant according to claim 10, wherein the integrated pulverizeris an impact hammer mill, whose rotor is arranged to spin a siftercoarse grain fraction back into the static cascade sifter for thepurpose of further pneumatic conveyance of the grinding product throughthe ascending duct to the subsequent sifter.
 12. A circulating grindingplant according to claim 7, wherein the static cascade sifter includes acoarse grain fraction discharge opening at its underside, said dischargeopening being connected to a material feed of the roller press via aconveying agent.